This invention relates to a process for the formation of an image using a positive photoresist and to supports bearing an image produced by this process.
In the production of an image using a positive photoresist, portions of the resist which are exposed to actinic radiation are rendered more soluble or more easily removable than the unexposed portions. The exposed portions can consequently be removed with a developer, leaving the unexposed portions intact, to form a positive image. Such image-forming processes are useful, for example, in the manufacture of printing plates and printed circuits.
Commercially available positive photoresists based on materials containing quinone diazides are not heat-stable. Furthermore, in the manufacture of printed circuits, they are not suitable for use both as a resist and as a dielectric material. For instance, in the manufacture of multilayer circuits using positive resists, it is desirable that the unexposed resist should be a good dielectric material so that there is no need to remove the resist before assembly of the layers. The commercially available positive resists are not suitable dielectric materials for use in such circuits.
Positive photoresists comprising polymers which depolymerise or photolyse on exposure to actinic radiation are known. For instance, U.S. Pat. No. 4,009,040 describes positive photoresists comprising depolymerisable condensation polymers containing hexa-arylbiimidazole groups. A paper by S. A. MacDonald, T. D. Schierling and C. G. Wilson, Organic Coatings and Plastics Chemistry Preprints (Amer. Chem. Soc.), 1980, 43, 264-7, describes photodegradable condensation polymers containing N-allyl-o-nitroaryl amide groups.
We have surprisingly found that resins or polymers containing certain substituted benzoin groups, although they show no increase in solubility when irradiated alone, become more soluble in a developer when irradiated in admixture with a monomer which is polymerisable under the influence of a free radical catalyst to form a higher molecular weight material which is more soluble in the developer than the mixture before irradiation. Such mixtures may therefore be used as positive photoresists. These resists are heat-stable. Furthermore, when the unexposed resin or polymer is heat-curable, the resist may be used in the manufacture of multilayer printed circuits without the need to remove the unexposed portion prior to bonding of the layers.